From 62024de44fcb9070c07f2944214afd21d3b9ba0c Mon Sep 17 00:00:00 2001
From: Roland Conybeare <rconybeare@gmail.com>
Date: Tue, 2 Dec 2025 10:37:07 -0500
Subject: [PATCH] xo-alloc / xo-ordinaltree: work on dual-alloc-policy trees

---
 include/xo/allocutil/IAlloc.hpp              |  6 ++++--
 include/xo/allocutil/IObject.hpp             |  3 +++
 include/xo/allocutil/gc_allocator_traits.hpp | 13 ++++++++-----
 3 files changed, 15 insertions(+), 7 deletions(-)

diff --git a/include/xo/allocutil/IAlloc.hpp b/include/xo/allocutil/IAlloc.hpp
index 4f9097f..a322e1d 100644
--- a/include/xo/allocutil/IAlloc.hpp
+++ b/include/xo/allocutil/IAlloc.hpp
@@ -76,7 +76,7 @@ namespace xo {
             std::byte * allocate(std::size_t n) {
                 return this->alloc(n);
             }
-                
+
             /** std::allocator locality hint. Not used for now **/
             std::byte * allocate(std::size_t n, const void * hint) {
                 (void)hint;
@@ -147,6 +147,8 @@ namespace xo {
             virtual void assign_member(IObject * /*parent*/,
                                        IObject ** lhs,
                                        IObject * rhs) { *lhs = rhs; }
+            /** if GC: evacuate @p *lhs and replace with forwarding pointer **/
+            virtual void forward_inplace(IObject ** lhs) { (void)lhs; }
             /** allocate @p z bytes for copy of object at @p src.
              *  Only used in @ref GC.  Default implementation asserts and returns nullptr
              **/
@@ -170,7 +172,7 @@ namespace xo {
             using const_reference = const T &;
             using size_type = IAlloc::size_type;
             using difference_type = std::ptrdiff_t;
-            
+
             using gc_object_interface = IAlloc::gc_object_interface;
             using has_incremental_gc_interface = IAlloc::has_incremental_gc_interface;
 
diff --git a/include/xo/allocutil/IObject.hpp b/include/xo/allocutil/IObject.hpp
index 87c6013..2104888 100644
--- a/include/xo/allocutil/IObject.hpp
+++ b/include/xo/allocutil/IObject.hpp
@@ -19,6 +19,9 @@ namespace xo {
      **/
     class IObject {
     public:
+        /** impl inheriting this class must provide gc hooks **/
+        static constexpr bool _requires_gc_hooks = true;
+
         /** true iff this object represents a forwarding pointer.
          *  Forwarding pointers are exclusively created by the garbage collector;
          *  forwarding pointers (and only forwarding pointers) return true here.
diff --git a/include/xo/allocutil/gc_allocator_traits.hpp b/include/xo/allocutil/gc_allocator_traits.hpp
index dd1a632..38e7f38 100644
--- a/include/xo/allocutil/gc_allocator_traits.hpp
+++ b/include/xo/allocutil/gc_allocator_traits.hpp
@@ -32,7 +32,7 @@ namespace xo {
          *       provide garbage collection
          *     - xo::gc::GC has a collection API and also provides
          *       garbage collection
-         *     
+         *
          *     GC-allocated objects must:
          *     2a. inherit A::object_interface
          *     2b. implement A::object_interface::_shallow_size()
@@ -40,7 +40,7 @@ namespace xo {
          *     2d. implement A::object_interface::_forward_children(alloc)
          *     in multiple inheritance scenarios
          *     2e. implement A::object_interface::_offset_destination(src)
-         *     
+         *
          *  Design Notes:
          *  - virtual-method choice requires vtable pointer per object;
          *    but zero *marginal* space cost for types that would have
@@ -82,12 +82,16 @@ namespace xo {
             //   gc_allocator_traits<Allocator>::template object_interface<Allocator>
             //
             template <typename A, typename = void>
-            struct object_interface {};
+            struct object_interface {
+                /** see also IObject::_requires_gc_hooks **/
+                static constexpr bool _requires_gc_hooks = false;
+            };
 
             // specialization when A provides gc_object_interface
             template <typename A>
             struct object_interface<A, std::void_t<typename A::gc_object_interface>>
-                : A::gc_object_interface {};
+                : public A::gc_object_interface {
+            };
 
             /** true iff this allocator advertises itself as an incremental collector
              *  allocator will include:
@@ -97,7 +101,6 @@ namespace xo {
              *  };
              **/
             static inline constexpr bool has_incremental_gc_interface_v = has_incremental_gc_interface<Allocator>::value;
-
         };
     } /*namespace gc*/
 } /*namespace xo*/